Atrial fibrillation (AF), which lay persons know as heart palpitations, is a commonly occurring cardiac arrhythmia. Generally, an AF episode is not life threatening, and the patient is functional during the episode. Some patients, however, feel under the weather, feel dizzy, or even lose consciousness during an AF episode. Nevertheless, even in the most severe cases, AF episodes without secondary sequelae and lasting less than 48 hours are thought to have no long-term adverse effects on a patient's health. Conversely, among other consequences, episodes lasting 48 hours or longer increase a patient's risk of stroke. Therefore, a patient's physician usually instructs him/her to seek medical treatment if an AF episode does not spontaneously terminate within 24 hours. This gives the patient sufficient time to actually receive treatment within the 48-hour safety window.
Referring to FIGS. 1 and 2, AF is characterized by irregularly distributed R—R intervals in a patient's electrocardiogram. FIG. 1 is portion of a patient's electrocardiogram that includes one R—R interval. The electrocardiogram includes P, Q, R, S, and T waves, and the R—R interval is defined as the interval between the upper peaks of adjacent R waves. FIG. 2 is a plot of the respective lengths of a patient's R—R intervals during an AF episode. In the electrocardiogram of a patient having a normal heart rhythm, the lengths of adjacent R—R intervals differ from one another by no more than a few milliseconds (ms), and thus are approximately equal. Therefore, during a period of normal heart rhythm, the plotted lengths of the R—R intervals would lie on or near the dashed line 10 in a normal distribution pattern. But during an AF episode, the plotted lengths of the R—R intervals differ significantly and randomly from one another. Therefore, during an AF episode, the plotted lengths 12 of the R—R intervals lie in a random distribution pattern with the appearance of a “bee swarm”.
There are several preventative and termination treatments available to patients with AF. Preventative treatments such as anti-arrhythmic drug therapy help prevent AF episodes from occurring, and termination treatments such as cardioversion terminate AF episodes once they have begun. As discussed below, some of these treatments are often expensive and/or inconvenient.
An external atrial defibrillator is a device that a cardiologist uses to apply one or more cardioverting electrical pulses, i.e., shocks, to the patient in order to terminate an AF episode. As discussed above, the cardiologist instructs his patient to notify the cardiologist's office if an AF episode lasts more than 24 hours. The cardiologist then admits the patient to the hospital on an in-patient or out-patient basis. While in the hospital, the patient is anesthetized and is shocked one or more times until the AF episode terminates. Unfortunately, this procedure costs approximately $1000–$5000 per session depending upon the procedure location within the hospital, and thus is relatively expensive. In addition, this procedure is burdensome to the patient for a number of reasons. For example, he/she often misses at least a day of work to undergo cardioversion. Furthermore, because the lingering effects of the anesthesia render him/her temporarily unfit to drive, the patient must find someone to drive him/her home from the hospital after the procedure. Because many AF patients require this procedure several times per year, the cumulative costs and burdens associated with this procedure can be quite substantial.
An internal atrial defibrillator is a device that is implanted within a patient's body and that applies one or more cardioverting electrical shocks directly to the patient's heart in order to terminate an AF episode. A manual model, such as the InControl Metrix, allows the patient to shock himself when he wishes to terminate an AF episode. In one known device, the patient initiates a shock by using a magnet to toggle a subcutaneous switch. Unfortunately, the implant surgery may cause discomfort to the patient, and complications such as infection may arise following surgery. Furthermore, additional surgeries can be required to replace the batteries or to repair or replace a defective unit. Alternatively, the internal defibrillator may include circuitry that detects an AF episode and automatically shocks the patient to terminate it. Unfortunately, in addition to the problems described above for the manual model, the automatic model may embarrass the patient. For example, a defibrillator shock affects not only the heart muscle, but often contracts most, if not all, of the voluntary muscles in the patient's thorax. Unfortunately, these contractions often cause the patient to “jump” uncontrollably. Therefore because the patient has no control over when the defibrillator delivers the shock, the shock, and thus this potentially embarrassing side effect, may occur during work or a social occasion.
Therefore, what is needed is an external atrial defibrillator that a patient or caretaker can use safely in the patient's own home.